Yet Another Joule Thief Tutorial




About: Maker, student in 10th grade. A Christian. Loves Jesus. Dreams to one day own a house that fully runs on renewable energy. Would like to visit the Maker Faire one day. Plans to be an electronic engineer (or ...

This is a tutorial for making the ever-popular LED Joule Thief. I made this instructable as a companion instructable

for my Joule Thief 10x10 LED Lamp.

Description of a joule thief from Wikipedia:

A joule thief is a minimalist self-oscillating voltage booster
that is small, low-cost, and easy to build; typically used for driving light loads. It can use nearly all of the energy in a single-cell electric battery, even far below the voltage where other circuits consider the battery fully discharged (or "dead").

A more understandable description:

The joule thief is a circuit that makes it possible to light up your typical LED's with 'dead' batteries. It does that by using the last bits of the power in those batteries, boosting it up to the voltage enough to make LED's light up.

So prepare your used batteries, and come with me to the journey of building the Joule Thief!

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Step 1: Preparing the Materials

One of the things you need to do before building something is preparing the materials. In this case most things we need can be salvaged easily. You can also buy them but building something from thrown away electronics is always more awesome.

The core part for the Joule Thief is the toroidal core. It is the thing that makes all this magic work!

The toroidal core can be found from the circuit board of a compact fluorescent lamp (CFL). It is normally found warped in some wires so remove them before using them here.

Warning: Be careful when taking the circuit boards from these lamps out. Don't break the tube of the lamp as it contains mercury vapour which is very poisonous.

After acquiring the toroidal core, you will need the magnet wire also known as the enamelled copper wire.

This wire can be found in transformers and is also used as inductors.

You will also need a 1kohm resistor and a NPN transistor. I used a 2N3904. 2N2222 and 2N4401 works too.

Now, after you have acquired everything you need to build a Joule Thief, let's go to the next step where you find out how to wind the wires to and to wire everything together.

Step 2: Winding the Magnet Wire.

Get about 60 cm of magnet wire and fold it in half. After that start winding the magnet wire starting from the connected ends. Try to keep the wires tidy so that it will look nicer in the end. Wind the wires around the toroidal core for about 8 to 11 turns. After winding the wires trim the ends leaving about 1cm or so for soldering later .(I left about 3 cm, it will be trimmed later during soldering) Sand the four ends of the wires to remove the enamel coating.

Now get your multimeter out (in continuity mode) and test the wires to find the ends of the 2 wires. If the wires are connected, then it is incorrect, you need to get the ends that are not connected. After confirming which one is not connected, twist them together so they are connected. Once they are connected you can proceed to test the circuit.

Step 3: Building and Testing Circuit

Based on the given schematic, wire the circuit on a breadboard or if you are confident you can solder them together.

The part where the wires are twisted together are supposed to be connected to the positive end of the battery and the other two ends are to be connected to a 1k resistor in series with the base of the NPN transistor (in my case it's the middle pin.) and the collector of the transistor.(right pin in my case.) The emitter of the transistor is then connected to the negative end of the battery. Lastly, the LED is to be connected to the collector and the emitter of the transistor. (anode - collector, cathode - emitter)

If the LED lights up, congratulations of finishing the circuit, but if it does not, check your wiring and windings and make sure they are correct. If you still can't get it to work, feel free to comment and I'll try me best to help you to make it work.

Step 4: Soldering

I build this Joule Thief for my LED nightlight so it is assembled inside s tight space in the battery holder I used. I modified the battery holder so all the batteries are connected in parallel and then I wire the circuit up, Most of the parts are heat-shrinked to prevent shorts so it might be a little hard to see where is some of the components.

Step 5: Complete

These are the LED nightlight I've made in another instructable. They are all powered by <1.5 volts. This makes them easy to be powered and you can use up most of your 'dead' batteries' power.

Do vote for me if you liked this tutorial.

Thanks for reading this and I hope you have learnt something....

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    13 Discussions


    3 years ago

    This is awesome.I like it.But I have doubt in this.This formula can be used for larger circuits.For ex....Mobile chargers,For tv,ac etc.....

    1 reply

    Reply 3 years ago

    You might be able to raise the voltage by sizing up the circuit but as the batteries can only supply very little power I don't think you would be able to do anything much with it. Maybe emergency lights? (Check


    For people who can't get it to work:

    Note that in the schematic, the dots on the transformer are on opposite ends. That means the two coils must be wound opposite.

    After you wind the coil, the "common end" can't be simply twisted together as the common point. You must cut it apart and wire one of the wires to another.

    Note in the picture how the twisted together wires come from opposite sides of the toroidal, and the other two wires also come from opposite sides. Use a continuity meter and make sure each "coil" is a single wire and not one end of each of the two.


    4 years ago on Step 2

    First, let me start off by saying I know virtually nothing about electronics or circuitry but I recently took an inexpensive solar garden light apart and found something called a Yx805 chip inside. Doing a google search on it, I find that it is essentially the important parts of your joule thief circuit, replacing the toroid and the transistor. Basically the garden light was the LED, the solar panel, a battery, the on/off switch, one resistor, and the Yx805 chip. Have you considered building your joule thief around a chip like this or another of the LED driver chips? If so, would you be so kind as to post the results?


    4 years ago on Step 3

    i used a huge 1.8mH's toroid instead :D with like 10 turns, it turns on an led from 1v but now lower, the weird thing is on my custom toroid it runs led at 3v when connecteed to anything higher than 3 ! and when i used a choke instead , it simply doesn't turn on !, also mine makes weird noises :D I wanted to use this to turn arduino on by 1 or 2 AA Batteries :|

    3 replies

    Reply 4 years ago on Step 3

    It's not the best idea to turn on an Arduino with this type of Joule
    Theif. This type was designed mainly for LED's and such, so If you want
    to make one that can power an Arduino, you need another type of
    circuit. As far as I know you need a few components such as an inductor,
    zener diode, some resistors and transistors.

    Check this instructable where ledartist used a different type of circuit to power his microcontroller from an AA battery.


    Reply 4 years ago on Step 3

    thanks for reply :)

    i will try to make it, but the zener i have is 35v :D i accidentally bought it :) i don't think i can find the parts easily, in my country its hard to find parts but thanks :)


    Reply 4 years ago on Step 3

    Can you buy from distributors such as Digikey, Element14 or RS components?


    4 years ago on Introduction

    i have tried this version before and couldn't get it to work. i found another version that worked. i may try this one again and see if i can get it to work.

    1 reply

    4 years ago on Introduction

    Ive used nearly all possible NPN transistors I can find and it works with all of them! I have gone a little bit joule crazy! I used a ton of random ones from old salvaged remotes for tvs etc.


    4 years ago

    Very nice! What is the minimum voltage this can operate?

    1 reply